Moving Picture Experts Group (MPEG) surround technology compresses multi-channel audio signals into downmix signals and side information. The MPEG surround technology can implement a decoder for the downmix signals and the side information bitstream transmitted from an encoder in either high-quality mode or low-power mode. A high-quality MPEG surround decoder provides high audio quality by using a residual signal and a temporal processing (TP) tool, but it requires a high degree of complexity. On the contrary, a low-power MPEG surround decoder reduces the complexity in such a method as changing computation of a Quadrature Mirror Filter (QMF) into a real number computation. Although the audio quality is somewhat degraded, the low-power MPEG surround decoder is appropriate for terminals consuming low power such as mobile phones.
An MPEG surround decoder restores downmix signals compressed with a general mono/stereo audio encoder, e.g., an Advanced Audio Coded (AAC) encoder or a High-Efficiency Advanced Audio Coding (HE-AAC) encoder, into multi-channel audio signals by using side information bitstream. Since the side information bitstream used herein is provided for each frequency band, the downmix signals should be converted to frequency bands using a hybrid filter bank, which consists of a QMF bank and a Nyquist filter bank. This conversion causes a delay. When the downmix signals are acquired after QMF processing of the HE-AAC decoder, as in the MPEG surround decoder, signals of the QMF domain can be directly extracted and applied to the MPEG surround decoder to thereby prevent delay caused by filtering.
The high-quality MPEG surround decoder can use not only the downmix signals of the time domain but also the downmix signals of the QMF domain that are acquired from the HE-AAC decoding process, as shown in FIG. 1. When the downmix signals of the time domain are used, a delay corresponding to 704 samples occurs in the process of executing a QMF analysis filter bank 101 and a Nyquist analysis filter bank 102. Also, a delay corresponding to 0 sample occurs in a Nyquist synthesis filter bank 201 and a delay corresponding to 257 samples occurs in the QMF synthesis filter bank 202 in the synthesis process of multi-channel audio signals shown in FIG. 2. In total, a delay corresponding to 961 samples occurs. When downmix signals encoded with an HE-AAC encoder are used, signals of the QMF domain that can be acquired from the HE-AAC decoding process can be directly used because the QMF of the high-quality MPEG surround decoder and the QMF of the HE-AAC decoder are identical. Also, since look-ahead signals corresponding to 384 samples needed for Nyquist banks is already available in a Spectral Band Replication (SBR) tool of the HE-AAC decoder, there is an advantage that no delay occurs in the filtering process.
However, when the downmix signals encoded with the HE-AAC encoder are applied to the MPEG surround decoder in the time domain, spatial parameters extracted from MPEG surround side information signals are delayed by 961 samples, including the delay corresponding to 257 samples occurring in the QMF synthesis process of the HE-AAC decoder and the delay corresponding to 704 samples occurring in the QMF filtering and the Nyquist filtering processes of the high-quality MPEG surround decoder. Thus, the downmix signals are synchronized between the HE-AAC decoder and the high-quality MPEG surround decoder to thereby be restored to desired multi-channel signals.
References, “ISO/IEC JTC1/SC29/WG11 N8177, Study on Text of ISO/IEC FCD 23003-1, MPEG Surround”, “Audio Engineering Society Convention Paper presented at the 115th Convention, Oct. 10 through 13, 2003, New York”, and “Audio Engineering Society Convention Paper presented at the 119th Convention, Oct. 7 through 10, 2005, New York” are incorporated herein.